Printed circuits with laser ablated conformal coating openings

ABSTRACT

Electrical components may be mounted on a printed circuit or other substrate. The electrical components may be covered with a conformal coating containing a wavelength-tuned-light-absorption-enhancement additive. The additive may be a dye or other additive that creates a light absorption peak at a given wavelength. To form openings in the conformal coating in alignment with the electrical components without damaging the components, a laser ablation tool may apply laser light at the given wavelength to the conformal coating. Openings may also be formed by placing tape over the components before the con form at coating is applied. The tape may have a color with a light absorption peak at the given wavelength to facilitate the formation of openings without damaging sensitive components.

This application claims the benefit of provisional patent applicationNo. 62/127,233, filed Mar. 2, 2015, which is hereby incorporated byreference herein in its entirety.

FIELD

This relates to electronic components such as components mounted onprinted circuits and, more particularly, to forming openings in printedcircuit conformal coatings.

BACKGROUND

Printed circuits are populated with integrated circuits, transducers,and other components. Printed circuits are mounted in the interior ofelectronic device housings. For example, printed circuits may be mountedin the interior of a cellular telephone or computer housing or otherelectronic device housing.

Some printed circuits are installed within the interiors of waterproofhousings. In sealed environments such as these, the risk of exposingsensitive components to moisture is minimal. In many situations,however, printed circuits are installed within housings that provide adegree of environmental protection, but that are not completelywaterproof. When installed in this type of device housing, there is arisk that sensitive components on a printed circuit may be exposed tomoisture during normal device operation.

To reduce the sensitivity of components on a printed circuit to exposureto moisture, the components can he protected with a conformal coating.The conformal coating may cover the surface of a printed circuit and mayprevent moisture damage by repelling liquids.

Conformal coatings may be deposited as blanket films. Effective sealingcan be achieved by using conformal coating materials that cover allexposed surfaces of the components on a printed circuit board, even inscenarios in which the components have uneven heights and a variety ofdifferent shapes. The ability of conformal coating to coat complexsurfaces without forming gaps allows the conformal coating to form asatisfactory environmental barrier, but can make it difficult to maskoff portions of the printed circuit board. It can therefore be difficultto form openings in the conformal coating for test points, electricalconnectors, and other components.

SUMMARY

Electrical components may be mounted on a printed circuit or othersubstrate. The electrical components may be covered with a conformalcoating containing a wavelength-tuned-light-absorption-enhancementadditive. The additive may be a dye or other additive that creates alight absorption peak at a given wavelength in the absorption spectrumof the conformal coating. To form openings in the conformal coating inalignment with the electrical components without damaging thecomponents, a laser ablation tool may apply laser light at the givenwavelength to the conformal coating.

Openings may also be formed by placing tape over the components beforethe conformal coating is applied. The tape may have a color with a lightabsorption peak at the given wavelength to facilitate the formation ofopenings without damaging sensitive components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view of an illustrative printed circuithaving electrical components covered by a conformal coating havingopenings in accordance with an embodiment.

FIG. 2 is an illustrative absorption spectrum for a conformal coatingwith an absorption-enhancing dye or other additive in accordance with anembodiment.

FIG. 3 is a diagram showing how a conformal coating may be deposited andpatterned using laser patterning techniques in accordance with anembodiment.

FIG. 4 is a diagram showing how tape may be used in facilitating laserpatterning of conformal coating material in accordance with anembodiment.

FIG. 5 is a flow chart of illustrative steps involved in depositing andpatterning conformal coatings in accordance with an embodiment.

DETAILED DESCRIPTION

Electronic devices may include components such as electrical componentsmounted on substrates such as printed circuits. An electronic devicethat includes one or more printed circuits may be a laptop computer, acomputer monitor containing an embedded computer, a tablet computer, acellular telephone, a media player, or other handheld or portableelectronic device, a smaller device such as a wrist-watch device, apendant device, a headphone or earpiece device, or other wearable orminiature device, a computer display that does not contain an embeddedcomputer, a gaming device, a navigation device, an embedded system suchas a system in which electronic equipment with a display is mounted in akiosk or automobile, equipment that implements the functionality of twoor more of these devices, or other electronic equipment.

The substrate onto which the electrical components are mounted may be arigid printed circuit board (e.g., a printed circuit formed from a rigidprinted circuit board material such as fiberglass-filled epoxy), may bea flexible printed circuit formed from a flexible sheet of polyimide orlayer of other flexible polymer, may be a rigid plastic supportstructures (e.g., molded or machined plastic), may be a dielectric suchas glass, ceramic, sapphire, or other dielectric material, or may be anyother suitable substrate. Illustrative scenarios in which electricalcomponents are mounted on a printed circuit board may sometimes bedescribed herein as an example. This is, however, merely illustrative.Any suitable substrate may be provided with electrical components, ifdesired.

The electrical components that are mounted on the substrate may includeintegrated circuits. For example, the electrical components may includestorage devices such as nonvolatile memory (e.g., flash memory or otherelectrically-programmable-read-only-memory memory configured to form asolid state drive), volatile memory (e.g., static or dynamicrandom-access-memory), or other storage circuits. The electricalcomponents may also include processing circuitry such asmicroprocessors, microcontrollers, digital signal processors, basebandprocessors, power management units, audio chips, application specificintegrated circuits, etc. The electrical components may include buttons,touch pads, key pads, keyboards, microphones, speakers, tone generators,vibrators, cameras, light-emitting diodes and other status indicators,connectors for data ports, board-to-board connectors and otherelectrical connectors, sensors such as an ambient light sensor and othersensors (e.g., a capacitive proximity sensor, a light-based proximitysensor, a magnetic sensor, an accelerometer, a force sensor, a touchsensor, a temperature sensor, a pressure sensor, a compass, a microphoneor other sound sensor, or other sensors), light-based components such aslight-based proximity sensors, image sensors, and other electroniccircuits and transducers.

A cross-sectional side view of an illustrative printed circuit that hasbeen populated with electrical components is shown in FIG. 1. As shownin FIG. 1, printed circuit 10 may have one or more layers of dielectricmaterial such as substrate 12 and one or more layers of metal traces 14in the dielectric material. Solder 20 and other electrically conductivematerial (e.g., conductive adhesive, welded metal, etc.) may be used inmounting electrical components 18 to metal traces 14 in printed circuit10.

To protect components 18 from the ambient environment (e.g., dust,moisture, chemicals, etc.), components 18 may be covered with aconformal coating such as conformal coating 22. Conformal coating 22 maybe formed from polymer or other materials that help protect components18. Examples of conformal coating materials include parylene,fluoropolymers, silicone, acrylic, epoxy, and polyurethane. Thesematerials may be deposited in an uncured state (e.g., as a liquid) andcured (e.g., using heat, room temperature curing, ultraviolet lightcuring, etc.). Other deposition techniques and conformal coatingmaterials may be used, if desired.

Openings such as openings 24 may be used to selectively exposecomponents 18. For example, components 18 may include connectors andother components with metal contacts. To clear the contacts so that theconnector can be attached to a mating connector, an opening such as oneof openings 24 may be formed over the surface of the contact. Metaltrace features such as contact pads 16 (e.g., test points for a testprobe or bond pads) may also be cleared by forming openings 24. In somesituations, components 18 may contain an optical component (e.g., atransparent optical structure). To ensure that the optical structureperforms properly, an opening such as one of openings 24 may he formedthrough layer 22 to expose the optical structure. In general, any typeof component 18 or structure on board 10 may be exposed by forming oneor more aligned openings 24.

Conformal coatings can penetrate between masking structures such as tapestructures and removable dams. Accordingly, it may be desirable to uselaser-based processing techniques (e.g., laser ablation techniques) todrill holes through layer 22. Conformal coating materials are oftenclear and exhibit minimal light absorption at laser wavelengths ofinterest. To enhance light absorption in conformal coating 22, alight-absorbing additive such as a dye or other additive may beincorporated in layer 22 (e.g., in the precursor material(s) used informing layer 22). For example, if it is desired to enhance theabsorption of layer 22 at green wavelengths, a green dye or otheradditive may be incorporated into coating 22 that enhances absorption oflayer 22 at green wavelengths. An illustrative absorption spectrum for aconformal coating is shown in FIG. 2. In the graph of FIG. 2, solid line30 corresponds to the absorption of conformal coating 22 without thewavelength-tuned-light-absorption-enhancement additive. When a dye orother additive with an absorption peak at wavelength λo is incorporatedinto coating 22, coating 22 will exhibit enhanced absorption 32. Theenhanced absorption peak may be a wavelength in the visible light rangeor may be an infrared or ultraviolet wavelength.

Illustrative equipment and techniques for forming openings in coating 22(i.e., a coating with a wavelength-tuned-light-absorption-enhancementadditive) are shown in FIG. 3. initially, component mounting equipment40 such as a pick-and-place tool or other soldering equipment may beused to mount components 18 on printed circuit substrate 12.

Conformal coating application equipment 42 may deposit conformal coating22 over components 18 on the surface of printed circuit substrate 12.Conformal coating 22 may be, for example, a liquid and equipment 42 maydeposit coating 22 using spraying, dipping, dripping, or needleapplication techniques. Deposition techniques such as physical vapordeposition and chemical vapor deposition and other techniques may alsobe used. Curing equipment (e.g., ultraviolet light curing equipment,heating equipment, etc.) may be used to facilitate curing of coating 22.The deposited layer may include a dye or otherwavelength-tuned-light-absorption-enhancement additive.

Laser equipment (laser ablation tool) 44 may be used to form openings24. Laser equipment 44 may include a computer-controlled positioner suchas positioner 46 and a laser such as laser 48. Laser 48 may be acontinuous wave (CW) laser or may be a pulsed layer. Laser 48 mayproduce light 50 at a visible wavelength, an ultraviolet wavelength, oran infrared wavelength. The wavelength of light 50 may match theabsorption peak of the additive in coating 22 to enhance absorption oflaser energy in layer 22 so that opening 24 may be ablated in layer 22.The enhanced absorption of light 50 helps prevent damage from light 50to underlying components 18.

If desired, a tape layer may be provided with a dye or other materialthat enhances absorption of light 50. For example, a green tape layermay be used in forming openings 24 when laser light 50 is green.Equipment involved with this type of approach is shown in FIG. 4.

As shown in FIG. 4, tape application equipment 52 may apply tape 54 overone or more components 18 on printed circuit substrate 12. Coatingequipment 42 may then be used to apply conformal coating 22. Conformalcoating 22 of FIG. 4 may include awavelength-tuned-light-absorption-enhancement additive or may betransparent. When exposed to laser light 50, light 50 will be stronglyabsorbed in tape 54, facilitating the formation of opening 24 in layer22 and tape 54 without damaging underlying components 18.

FIG. 5 is a flow chart of illustrative steps involved in formingopenings 24 in conformal coating 22.

At step 60, equipment 40 may be used to mount electrical components onprinted circuit 18. Printed circuit 18 may also contain metal tracesthat form test pads and other contacts for which it may be desired toform conformal coating openings 24.

If it is desired to incorporate tape 54 within the board, equipment 52may be used to apply tape 54 (step 62).

At step 64, the printed circuit board (with or without the tape) iscoated with conformal coating using tool 42. The conformal coating mayinclude a wavelength-tuned-light-absorption-enhancement additive toenhance absorption of laser light 50. Laser light 50 may be used toablate openings in layer 22 at step 66 using the additive in layer 22and/or tape 54 to promote light absorption without damaging underlyingstructures.

After forming openings 24 by exposing layer 22 (and, if desired,underlying tape layer 54) to light 50, device assembly may be completed(step 68). For example, electrical contact may be formed with exposedmetal structures, printed circuit board 10 may be mounted in anelectronic device, etc.

The foregoing is merely illustrative and various modifications can bemade by those skilled in the art without departing from the scope andspirit of the described embodiments. The foregoing embodiments may beimplemented individually or in any combination.

What is claimed is:
 1. Apparatus, comprising: a substrate; an electricalcomponent mounted on the substrate; and a conformal coating on thesubstrate with a wavelength-tuned-light-absorption-enhancement additiveand a laser-ablated opening through the conformal coating, wherein thelaser-ablated opening directly overlaps the electrical component andwherein the wavelength-tuned-light-absorption-enhancement additive has alight absorption peak at a wavelength chosen from the group ofwavelengths consisting of: an ultraviolet light wavelength, a visiblelight wavelength, and an infrared light wavelength.
 2. The apparatusdefined in claim 1 wherein the conformal coating is selected from thegroup consisting of: acrylic, epoxy, silicone, polyurethane, parylene,and fluoropolymer.
 3. The apparatus defined in claim 2 wherein thewavelength-tuned-light-absorption-enhancement additive has a lightabsorption peak at an ultraviolet light wavelength.
 4. The apparatusdefined in claim 2 wherein thewavelength-tuned-light-absorption-enhancement additive has a lightabsorption peak at a visible light wavelength.
 5. The apparatus definedin claim 2 wherein the wavelength-tuned-light-absorption-enhancementadditive has a light absorption peak at an infrared light wavelength. 6.The apparatus defined in claim 1 wherein the substrate comprises aprinted circuit.
 7. The apparatus defined in claim 6 wherein thewavelength-tuned-light-absorption-enhancement additive has a lightabsorption peak at a visible light wavelength.
 8. The apparatus definedin claim 6 wherein the printed circuit has a contact pad and wherein theopening overlaps the contact pad.
 9. The apparatus defined in claim 8wherein the wavelength-tuned-light-absorption-enhancement additive has alight absorption peak at a visible light wavelength.
 10. The apparatusdefined in claim 1 wherein thewavelength-tuned-light-absorption-enhancement additive comprises a dye.11. The apparatus defined in claim 10 wherein the dye has a lightabsorption peak at a visible wavelength or at an infrared wavelength.12. The apparatus defined in claim 11 wherein the substrate comprises aprinted circuit, wherein the printed circuit has a contact pad, whereinthe electrical component is mounted on the printed circuit, and whereinthe opening overlaps.
 13. Apparatus, comprising: a substrate; acomponent mounted on the substrate; tape that absorbs laser light at alaser wavelength that is provided by a laser ablation tool, wherein thetape has a color with a light absorption peak at the laser wavelength;and a conformal coating on the substrate that overlaps at least some ofthe tape, wherein there is an opening through the conformal coating andthe tape that exposes at least a portion of the component.
 14. Theapparatus defined in claim 13 wherein the conformal coating is selectedfrom the group consisting of: acrylic, epoxy, silicone, polyurethane,parylene, and fluoropolymer.
 15. The apparatus defined in claim 13wherein the opening through the conformal coating directly overlaps thecomponent.
 16. The apparatus defined in claim 15 wherein the openingextends completely through the conformal coating and the tape.
 17. Theapparatus defined in claim 16 wherein the tape and the conformal coatingoverlap the printed circuit board and the component and wherein there isan additional opening that exposes at least a portion of the printedcircuit board.
 18. The apparatus defined in claim 13 wherein theapparatus comprises an interior and an exterior, wherein the tape andthe conformal coating separate the interior from the exterior, andwherein the opening through the conformal coating and the tape exposesthe component to the exterior of the apparatus.
 19. Apparatus,comprising: a substrate; a conformal coating on the substrate with awavelength-tuned-light-absorption-enhancement additive and alaser-ablated opening through the coating, wherein thewavelength-tuned-light-absorption-enhancement additive has a lightabsorption peak at a wavelength chosen from a group of wavelengthsconsisting of: an ultraviolet light wavelength, a visible lightwavelength, and an infrared light wavelength.
 20. The apparatus definedin claim 19 wherein the apparatus has an interior and an exterior andwherein the conformal coating separates the interior from the exterior,the apparatus further comprising: an electrical component mounted on thesubstrate, wherein the laser-ablated opening through the conformalcoating exposes the electrical component to the exterior of theapparatus.